1. Field of the Invention
This invention relates to fibrous mineral wool products. More particularly, it relates to a method for manufacturing strong, structural panels of mineral fiber having a density of from about 3 to about 10 pounds per cubic foot which may be used as acoustical ceiling tiles, thermal insulating panels, sound absorbing panels, pipe and beam insulation and similar products.
2. Description of the Prior Art
The water felting of dilute aqueous dispersions of mineral wool and lightweight aggregate is known. By such methods, a dilute dispersion of mineral wool, lightweight aggregate, binder and other adjuvants are flowed onto a moving foraminous support wire screen such as that of an Oliver or Fourdrinier mat forming machine for dewatering at line speeds of about 10-50 feet per minute. The dispersion dewaters to form a mat first by gravity means and then by vacuum suction means. The wet mat is dried over a number of hours in convection drying ovens; and the product is cut and optionally top coated to produce lightweight structural panels such as acoustical ceiling tiles. Such methods cannot produce low density structural panels below about 12 pounds per cubic foot density. A "structural" panel, by definition, is capable of supporting its own weight without visible sagging, bending or collapsing when supported only at the edges of the panel, as in a suspended ceiling grid.
It is also known to form stable foams with mineral wool. U.S. Pat. No. 4,447,560 suggests a low density insulation sheet may be made by forming a first slurry of fiber containing synthetic rubber latex solids. A detergent slurry is then formed and the two slurries admixed to about 15% solids consistency, agitated to a stable foam, and oven dried. The extremely time consuming, and energy intensive, drying of the stable foam from 15% solids is a severe economic detriment.
U.S. Pat. No. 4,613,627 discloses a modified wet pulp process for forming an acoustical ceiling tile wherein the binder is foamed separately from the rest of the solid ingredients. The foamed binder is then combined with an admixture of the other solids, and the admixture is cast, screeded, textured, press molded and dried.
The use of foam to prevent stratification of the various particles in a slurry of mineral wool, aggregate and other solids during the water felting of mineral fiber panels is taught by Guyer et al in U.S. Pat. No. 4,062,721. The foam retains the particles in a space matrix but also increases the water drainage time according to Guyer et al who solve that problem by delaying the foaming of the furnish until after gravity drainage has occurred. Guyer et al teach that more water is removed because the foam reduces the gross porosity of the furnish thus making vacuum dewatering more effective. This means that air is not passing through the furnish but pressing down on it and reducing the porosity still further.
Bryant teaches in U.S. Pat. No. 1,841,785 that a tough coherent skin of paper-like consistency may be created on the lower surface of a foamed mass of cellulose fibers and water on a Fourdrinier wire by subjecting the lower surface momentarily to a vacuum without imparting the suction deeply into the mass so that only the lower surface area is compacted. Further dewatering of the foamed mass occurs under a lesser vacuum so that the fibrous body of the mass is not broken down or compacted. The still wet fibrous body is then dried by passing it through an oven into which hot air is blown at levels above and below the fibrous body. The spongy consistency of the body, except for the tough skin, is thus preserved.
European Patent Application No. 148,760 teaches the manufacture of an air permeable sheet of mineral fibers and plastic powders. A dispersion of glass fibers, plastic powder, and a foaming agent is aerated to produce a fine-bubbled foam which is then drained on Fourdrinier wire to form a web of unbonded fibers interspersed with the plastic. The web is transferred carefully from the wire to a mesh belt where a binder is applied to the web and it is dried gently in a drying tunnel whereupon bonding of the mineral fibers takes place. Some loss of the plastic powder occurs.
British Patent Specification No. 1,263,812 teaches a method for forming a fiber-containing polymeric sheet capable of being thermoformed or cold pressed. The method includes feeding a paste of a polymeric powder and binder into a foamed suspension of fibers, dewatering the foamy mixture and drying the resulting sheet on a rotary drum drier. The foam is maintained by the addition of surfactant as needed. The dried sheet has a soft, crumbly texture.
Furthermore it is known that paper webs constituted mainly by noble cellulose fibers and fibrilles may be formed from foams. The basic formation of the cellulose fiber for manufacture of paper gives rise to highly fractured fiber fragments and fibrilles having jagged, fuzzy, microstructured surfaces suitable to trap and aid entanglement of microscopic-sized foam bubbles. This is not true for mineral fibers or mineral aggregate, which have smooth surfaces in comparison to the jagged and fuzzy microstructured surfaces of cellulose fibers.
In U.S. Pat. No. 3,228,825, it is suggested that lightweight foams of attenuated glass fibers might be formed into lightweight fibrous products of less than 5 pounds per cubic foot density. According to this patent, microscopic bubbles are generated in an aqueous suspension of lightweight aggregate and attenuated glass fibers in order to achieve uniform incorporation of both in the fibrous structure. Very highly refined cellulosic fibrilles serve as the binder for the glass fibers. The proposed products would appear to be extremely flexible, incapable of structural panel uses.
It is an object and advantage of the present invention to provide low density structural panels of mineral fiber by a wet felting process without having to dry extremely high amounts of water out of the wet mat over long periods of time.
A further object is to provide panels having excellent strength and integrity at densities less than about 10 pounds per cubic foot.
A still further object and advantage is the provision of a wet felting method for manufacturing low density mineral panels in a facile, rapid manner wherein the mat is dewatered and dried in a few minutes.
The above objects and advantages, and others which will become more apparent from the ensuing description, are based upon the peculiar rheology of a delicate, aqueous froth of unstable and weak bubbles, and further upon high volume, high velocity through-air drying of wet, open porous structures. Basically, in accordance with the present invention, the applicant has now discovered a process for rapidly forming shaped structural panels such as acoustical ceiling tile that combine very low densities with good strengths. A modified wet process is employed wherein a dilute aqueous slurry is foamed to a delicate froth between scrim cover sheets on a moving foraminous support wire screen. The froth dewaters and matures under quiescent conditions and is then rapidly ruptured by high vacuum to form a sufficiently stable porous structure that may be rapidly stripped of remaining water and dried by passing large volumes of heated air through the structure without any substantial collapse of the open porous structure.